A known device of this kind is disclosed in the German Patent Specification No. 28 07 866. It comprises a volumetric dosing unit, said dosing unit comprising a rotatable dosing disc having a vertical axis of rotation, a drive member arranged below said dosing disc, as well as a powder container having an outlet for laying down powder onto said dosing disc and a powder take-up unit operating under gas pressure for taking up powder from said dosing disc, said powder container and said powder take-up unit being arranged above said dosing disc.
For working powder in a plasma spray device, an exact reproducible dosing and an even powder supply are essential. A predetermined amount per time unit of powder is to be supplied to the plasma spray device. This demand, in principle calls for a gravimetric method for dosing the powder. However, so far, in most cases the closing was achieved by a volumetric dosing, thereby calibrating the dosing unit by weight by weighing a powder portion, destined for being supplied within a certain period of time. The volumetric dosing units actually available, such as e.g. those according to said German Patent Specification No. 28 07 866, have a very good dosing accuracy. However, they show the disadvantage that calibration is to be repeated after each removal of the powder charge.
In order to avoid this disadvantage, it was already proposed to mount the dosing unit on a balance, and to control the speed of the drive motor of the dosing unit during the powder withdrawal, starting from the initial total weight of the dosing unit plus the powder stock, i.e to control the volumetric dose on the basis of the measured loss of weight.
However, it turned out that such an arrangement is quite unstable, and that in operation the dosing unit has a liking for oscillations, thus considerably reducing the dosing accuracy which could be theoretically expected on the basis of the accuracies of the balance and of the dosing unit, and possibly resulting in an irregular powder supply. The main reason for this is that the dosing unit mainly and foremost extends in vertical direction, i.e. that the dosing unit is relatively tall, form the lower end of the drive to the upper end of the powder container, as compared to the diameter of the dosing unit which is essentially determined by the lateral extension of the dosing disc. With known dosing units of the abovementioned kind, this ratio of height to diameter may be up to 5:1, and more.